1. Field of Invention
The invention relates generally to an antenna of a radio device, such as small-sized mobile wireless terminals, and particularly in one exemplary aspect to monopole antenna and related structures.
2. Description of Related Technology
In small-sized radio devices, such as mobile phones, the pursuit of space saving is a significant factor when designing a device. Regarding the antenna, using a dielectric substrate for the radiator, using a monopole radiator instead of PIFA (Planar Inverted-F Antenna), and the above-mentioned implementation of the radiator in the outer cover of the radio device are commonly used solutions which save space.
Making the radiator as a part of the device's outer cover is known. An example of such an arrangement is the solution for the antenna of a radio device shown in FIG. 1, known from the publication EP 1 439 603. Therein, a radio device can be seen from behind and its cross section at the antenna structure. The upper part 130 of the rear part of the device's outer cover is comprised of conductive material and functions as the radiator of the antenna. The radiator 130 joins the dielectric rest part 150 of the outer cover without discontinuity.
In the cross section shown in FIG. 1 there is seen the radiator 130, and below it the circuit board PCB of the device, on the upper surface of which board the ground plane GND of the antenna is located. Between the radiator and ground plane there is a conductive feed element 120 which is separated galvanically from the radiator by a distinct thin dielectric layer DIE. The radiator has no galvanic coupling to any conductive part of the radio device. Instead, the feed element 120 is coupled galvanically to the antenna port of the radio device by the feed conductor 116 and to the ground plane by the short-circuit conductor 115. An example of the shape of the feed element 120 is visible in the drawing of the radio device. It is a conductive strip comprising, as viewed from its short-circuit point SP, two arms with different lengths to implement two operating bands for the antenna. The longer arm of the feed element, the radiator 130, and the ground plane GND constitute a resonator, the natural frequency of which is in the lower operating band, and the shorter arm of the feed element, the radiator and the ground plane constitute a resonator, the natural frequency of which is in the higher operating band.
The use of the separate feed element is favourable, because in that case the placement of the antenna's operating bands and the matching of the antenna can be arranged without interfering with the shape of the radiator. However, a flaw of the solution in FIG. 1 is that good band characteristics and adequate efficiency require a remarkably long distance between the radiator and the ground plane GND. This again means that the need for antenna space is also in this case greater than desirable, thereby making the overall device size larger.
An object of the invention is to implement an antenna provided with a cover radiator in a new and advantageous way.
In one exemplary embodiment of the invention, radiator of an antenna is trough-like so that the head surface, front and rear surfaces and both side surfaces of the dielectric cover of a device are coated by it at an end of the radio device. On the side of the side surfaces there are slots in the radiator starting from its edge for increasing the electric size. The radiator is fed electromagnetically by a separate element which is shaped so that the antenna has at least two operating bands. The ground plane of the antenna is apart from the radiator, thus not extending inside the ‘trough’.
An advantage of the invention is that the radiator of the exemplary antenna is short in the longitudinal direction of the radio device, and the space required by the antenna in the radio device is thus relatively small. This is due to the trough-like shape of the radiator and the slots on its sides. Thanks to the slots the radiator, which resonates at a certain frequency, becomes smaller. Also the monopole structure, or the location of the ground plane apart from the radiator, results in a smaller size of the radiator. Another advantage of the invention is that several resonances, which can be tuned separately, are provided for the antenna.
In another aspect of the invention, an antenna for use in a radio device is disclosed. In one embodiment, the antenna includes: a feed element having feed and short circuit points associated therewith, at least the short circuit point being in electrical communication with a ground plane of the radio device; and a device end cover, the end cover comprising a dielectric and having the feed element disposed on an interior surface thereof, and at least one conductive coating on an outer surface thereof, the at least one conductive coating forming at least part of a multiband radiator.
In another embodiment, the radio device has a head surface and first and second side surfaces, and the antenna comprises: a radiator having an antenna cover associated therewith, a feed element comprising an electromagnetic coupling to the radiator; and a ground plane in electrical communication with the radiator. In one variant, the antenna has at least a lower operating band and a higher operating band; the radiator is disposed substantially at an end of the radio device, and comprises a conductive coating of a dielectric cover of the radio device, the dielectric cover comprising a trough-like shape so that its bottom corresponds to the head surface of the radio device; the antenna cover is a part of the dielectric cover, and the radiator comprises a slot proximate at least one of the side surfaces and starting from an edge of the radiator and to decrease its physical size at the frequencies of the lower operating band of the antenna.
In a further aspect of the invention, a mobile wireless device is disclosed. In one embodiment, the device includes: a radio transmitter and receiver; a first device housing element; a circuit board disposed substantially with the first device cover, the circuit board comprising a ground plane; and a dielectric device end housing element comprising: a feed element having a short circuit point in communication with the ground plane, and a feed point coupled to the radio transmitter and receiver; and a slotted radiator formed on at least part of an exterior of the end housing element. The feed element, short circuit point, ground plane, and slotted radiator cooperate to provide the wireless device with multiband wireless capability.
In another aspect of the invention, a method of operating a wireless antenna is disclosed. In one embodiment, the antenna comprise a feed element with feed point, a short circuit point, a dielectric element, and a radiating element, and the method includes: feeding a signal to the feed element via the feed point; short circuiting the feed element to a ground plane via the short circuit point; and electromagnetically coupling at least portions of the feed element to the radiating element via the dielectric element, the coupling causing the radiator to radiate at least portions of the signal in at least one of an upper band and a lower band in which the antenna is capable of operating.
FIG. 1 was already described in connection with the description of prior art.
FIG. 2 shows an example of the radio device provided with an antenna according to one embodiment of the invention. The radio device RD is elongated and relatively flat so that it has two ends, head surfaces corresponding to these ends, front and rear surfaces, and first and second side surfaces. The antenna radiator is located at one end of the device which can be the end on the side of the device's speaker or the opposite end. In the figure is shown the outer cover of the device which comprises the main part COV of the cover and the antenna cover 332. In this example, the antenna cover is of conductive material, for which reason it functions as a part of the radiating structure. The radiator proper is disposed underneath it. Also, the main part COV of the cover is in this example of conductive material, for which reason there is a relatively narrow non-conductive part SPL of the outer cover between it and the antenna cover 332.
The antenna cover of the embodiment of FIG. 2 is trough-like by shape, opening naturally towards the middle part of the radio device. A slot, such as the first slot SL3, is formed on the side of the both side surfaces of the antenna cover, starting from its edge. These slots are discussed in greater detail below with respect to the description of FIG. 3.
FIG. 3 shows an example of the structure according to the invention. The antenna of the radio device presented in FIG. 2 is used as the example. The structure comprises dielectric cover 310, feed element 320, radiator 331 and the above-mentioned antenna cover 332. For the sake of clarity, the antenna cover has been drawn as separate in the figure; in the assembled device it is disposed over the other parts of the antenna structure.
The dielectric cover 310 forms the frame of the end of the radio device and, at the same time, the antenna frame. It is trough-like by shape so that it has a bottom, opposite short sides and opposite long sides. The bottom corresponds to the head surface of the whole device, the short sides correspond to the side surfaces of the whole device at the device's end in question, and the long sides correspond to the front and rear surfaces of the whole device at the device's end. On the outer surface of the dielectric cover 310 there is disposed the radiator 331. This has, in the illustrated embodiment, been implemented by metallizing the outer surface e.g. with the LDS method (Laser Directed Structuring). Therefore, the radiator is trough-like following by shape the head surface, the front and rear surfaces and the side surfaces of the radio device. The areas of the outer surface of the dielectric cover starting from the edges of its short sides are left without metallizing so that on the side of the side surfaces of the radio device, there are slots directed from the edge of the radiator towards its bottom. On the side of the first side surface there is the first slot SL1, and on the side of the second side surface there is the second slot SL2. These slots increase the electric size of the radiator. As a result, the resonance, which corresponds to the lowest operating band of the antenna, can advantageously be realized by means of a physically smaller radiator compared to a radiator without said slots.
The width of the radio device determines the length of the radiator, in which case the decrease in size means the shortening of the radiator in the longitudinal direction of the device, or in the depth direction of the radiator.
On the inner surface of the dielectric cover 310 there is disposed the feed element 320 of the radiator. In the illustrated example it extends in the depth direction from the open edge of the dielectric cover to the bottom surface of the cover. The feed element is connected to the transmitter and receiver of the radio device from its certain point FP, which is the feed point of the antenna. Only an electromagnetic coupling exists between the feed element and the radiator. The feed element is shaped so that it has several resonances together with the radiator and ground plane. The operating bands of the dualband antenna are based on these resonances. An example of the foregoing is shown in FIGS. 4 and 5. The feed element is connected also to the ground plane of the antenna from its short-circuit point SP.
In the finished structure, the antenna cover 332 is tightly placed on the radiator. If the antenna cover is formed of conductive material, as in this example, slots are needed on the side surfaces of the device in order to prevent the short-circuit of the slots SL1, SL2 of the radiator. The third slot SL3 in the antenna cover is formed at above-mentioned first slot SL1, and the fourth slot SL4 is formed at the second slot SL2. If the radio device has a steel cover, also the antenna cover is formed of steel for the sake of appearance. As a conductor, steel is clearly poorer than the metal which is used as the radiator 331. For this reason, an antenna cover made of steel has little significance as a radiator, although it in principle functions as a part of the radiating structure. The antenna cover can also be made of a non-conductive material, in which case it may lack the slots SL3 and SL4 seen in FIG. 3.
FIG. 4 shows an example of the feed element of an antenna according to one embodiment of the invention, as if spread on a plane. Also, the ground plane GND of the antenna is visible in the figure, and is located beside the feed element (and thus also the radiator 431 as viewed in the direction of the normal of the ground plane). The ground plane is in this embodiment parallel with the front and rear surface of the radio device.
The feed element 420 includes the short-circuit point SP and the feed point FP of the whole antenna, and it includes in the illustrated embodiment three parts. The first part 421 comprises six successive portions which form a simple spiral pattern: the first portion of the first part starts from the feed point FP and is directed away from the ground plane, that is, towards the bottom of the radiator. The second portion is parallel with the edge of the ground plane, or perpendicular to the first portion. The third portion is directed back towards the ground plane, the fourth portion towards the first portion, the fifth portion towards the second portion and the sixth portion towards the third portion. In the tail end TE of the sixth portion, there is an extension towards the second portion for strengthening the electromagnetic coupling between the tail end and the second portion.
The second part 422 of the feed element 420 is a mainly straight conductor strip between the ground plane GND and the spiral formed by the first part 421. The second part starts from the feed point FP, and ends at the point from which the feed element is connected to the ground plane, or the short-circuit point SP. In the example of FIG. 4, there is included a capacitive element C41 between the feed point and the starting end of the second part. By means of the capacitance, the antenna matching in the lower operating band is improved so that this band is widened.
The third part 423 of the feed element of the illustrated embodiment is a continuation of the second part from the short-circuit point SP forward. It has the same direction as the second part, and is open at its outer end.
The exemplary feed element 420 has three significant resonances together with the radiator and ground plane. The lowest frequency resonance, or the first resonance, is based on the electric length of the structure constituted by the first 421 and second 422 part of the feed element and the capacitive element between these parts. The higher frequency resonance, or the second resonance, is based on the electric length of the conductor constituted by the second 422 and third 423 part. It can be tuned by changing the physical length of the third part. The highest frequency resonance, or the third resonance, is a harmonic resonance of the first resonance. It can be tuned by changing the electromagnetic coupling between the second and sixth portions of the first part 421 by shaping the tail end TE of the sixth portion.
Also, a tuning element 440 is seen in FIG. 4. It is in one embodiment comprised of a conductor strip on the inner surface of the dielectric cover, connected to the ground plane from its one end. The tuning element is used to affect the resonance which the radiator has alone (naturally with the co-operation of the ground plane), which resonance is then the fourth significant resonance of the antenna to be considered. The tuning element can be located, in respect of the feed element, for example, on the opposite side of the trough formed by the dielectric cover.
FIG. 5 shows an example of the band characteristics of an exemplar, antenna configured according to the invention. The antenna used as the basis of FIG. 5 is comparable to that presented in the previous figures; the capacitance of the matching capacitor C41 between the feed point and second part of the feed element is 4.7 pF. The curve shows the fluctuation of the reflection coefficient S11 of the antenna as the function of frequency. The lower the reflection coefficient, the better the antenna has been matched. The feed element is shaped so that the antenna has two operating bands, the lower and higher one. The lower operating band is based on the above-mentioned first resonance r1, the frequency of which is about 0.9 GHz. It is seen from the curve that the lower operating band covers the frequency range 880-960 MHz (w1 in the figure) used by the EGSM system (Extended GSM), when the value −5 dB of the reflection coefficient is considered to be the criterion of the boundary frequency of the band. The higher operating band is based on the above-mentioned second, third and fourth resonance. The frequency of the second resonance r2 is about 1.75 GHz, the frequency of the third resonance r3 is about 2.1 GHz and the frequency of the fourth resonance r4 is about 2.0 GHz. It is seen from the curve that the higher operating band well covers the frequency range 1710-2170 MHz (w2 in the figure) used by the systems GSM 1800, GSM 1900 and WCDMA in all. The fourth resonance is tuned between the second and third resonances on the frequency scale, by which means the reflection coefficient can be kept low in the entire large operating band. Without the fourth resonance, the antenna matching would be poor in the middle range of the band.
FIG. 6 shows an example of the efficiency of an antenna in free space according to one embodiment of the invention. Curve 61 shows the fluctuation of the efficiency in the lower operating band. In the range w1, the efficiency fluctuates between −1 dB and −3 dB, being about −1.6 dB on average. Curve 62 shows the fluctuation of the efficiency in the higher operating band. In the range w2, the efficiency fluctuates between −0.5 dB and −3.7 dB, being about −2.4 dB on average. The values of the efficiency are accordingly quite good.
An adjustable antenna according to the invention has been described above. In details, its structure can naturally differ from what is presented. For example, the shape of the feed element of the antenna can vary widely. The resonance frequencies of the antenna can be arranged also so that the number of the operating bands is more than two. The feed element can also be located on the surface of a separate thin and flexible dielectric plate which is fastened on the surface of the antenna frame. The antenna frame again can be, besides the same as the dielectric cover of the antenna, also a separate dielectric object to be placed inside the dielectric cover. The tuning element affecting the resonance frequency of the radiator can be located at different places in the antenna structure. The inventive idea can be applied in different ways within the scope of the invention as set forth in the various exemplary embodiments and disclosure provided herein.